Magnetic resonance imaging (MRI) is a technique for producing high resolution images of the interiors of samples under study. One application is the imaging of human bodies: MRI machines can produce detailed, cross-sectional images of patients. The images, or slices, can be digitally "stacked" together to form three dimensional views of internal organs and tissue structures. It allows doctors to look inside human bodies without cutting. MRI is one of the most important medical diagnostic tools today.
Most elements have at least one abundant isotope whose nucleus is magnetic. Because such a nucleus is dipolar, it behaves as a very small magnet. When the nucleus is placed in a strong external magnetic field, it will be aligned with the field. A weak but rapidly alternating magnetic field alters the orientation of the nuclei, which causes it to absorb energy. When the nuclei realigns itself with the strong field and returns to equilibrium, it emits energy which can be detected by an antenna placed nearby. This precession is continuously repeated during the application of the alternating magnetic field. The resonance frequency of each nuclear species is determined by its unique precessional properties, and the strength of the magnetic field. This is expressed in the formula .omega.=.gamma.H, where .omega. is in radians per second, .gamma. is the gyromagnetic ratio characteristic of the nuclear species, and H is the magnetic field. Therefore, all magnetic nuclei of the same species, e.g., hydrogen nuclei in tissue water, will have the same resonance frequency at a particular field strength, regardless of their spatial position in a sample. These principles form the basis of MRI.
A conventional MRI machine includes the following basic components: (1) a ring-shaped enclosure housing a very large magnet capable of producing a very strong, homogeneous magnetic field on the order of 1 to 2 Tesla. A homogeneous field is one in which the field strength is substantially the same at all points within the volume of interest; (2) radio frequency gradient coils for producing weaker, linear gradient fields in each of the three main axes. A linear gradient field is one in which the field strength varies in a linear manner along a particular axis; (3) a sensor coil for detecting emissions as the nuclei precess; and (4) a computer with a display for producing an image bases on the received signals.
An object to be studied, e.g., a human, is placed in the homogeneous or main field within the hole in the enclosure. When a linear gradient field is added, nuclei at different positions within the object will experience different magnetic field strengths, and therefore will precess at different resonance frequencies. The signals given off are detected by the sensor coil as complex wave forms, which are converted to frequency spectra by the computer using Fourier transformation. The resonance frequency of the signal from a nucleus is proportional to the field strength, which in turn is proportional to the distance between the point of interest and the gradient coil. Therefore the position of the nucleus can be determined by imposing gradients in different directions and detecting the frequencies of the signals.
Conventional MRI machines used in medical applications are designed to image entire sections of humans, e.g., to produce images of a brain or part of a torso, The magnets are large enough to surround their subjects, and produce extremely strong fields. Permanent magnets are not suitable for such machines, because when sized for humans, they can weight as much as 113,700 kg (250,000 lb.). Superconducting magnets are generally used because their electrical efficiency allows them to produce extremely strong, homogeneous fields at human-size apertures, and their stability is excellent.
Although conventional MRI machines are powerful tools, they have some disadvantages. Their superconducting magnets are expensive to produce, and they require cryogenic cooling. Their extremely powerful fields attract ferrous objects from many meters away, which can become dangerous projectiles. The large size of the magnets and the ancillary equipment fill whole rooms, so that they are limited to operation at fixed sites. They are also very expensive to operate.
What is needed is a hand-held instrument compatible with a general-purpose computer, capable of imaging small portions of anatomy, such as the human eye, and of producing images of the scanned volume on the display of the general-purpose computer.